1. Field of the Invention
The present invention relates generally to a polishing apparatus and more particularly to a polishing apparatus configured to polish both surfaces of a work. For example the present invention may be applied to a Chemical Mechanical Polishing or Planarization (“CMP”) polishing apparatus.
2. Description of the Related Art
A Micro Electro Mechanical System (“MEMS”) sensor is one example of MEMS and needs to be maintained in a vacuum environment by bonding a glass substrate to both sides of a MEMS chip having a sensing function. Accordingly, the MEMS chip side of the glass substrate needs to have a high degree of flatness. The manufacture becomes more convenient when the front and back surfaces of the glass substrate are not distinguished during manufacturing. For these reasons, there is a demand to polish both the front and back surfaces of each glass substrate with the same degree of flatness.
A polishing process includes a finishing (rough lapping) step that roughly laps a surface with a surface roughness RA between 1 μm to 200 nm, and a super finishing step that highly precisely laps the surface with a surface roughness Ra of several nanometers. Japanese Patent Application, Publication No. (“JP”) 2000-305069 proposes use of a CMP apparatus for the super finishing step. A conventional CMP apparatus requires a glass substrate to be detached, reversed, and mounted again, after one surface of the glass substrate is polished, in order to polish both surfaces of the glass substrate.
Simultaneous polishing of both surfaces of the substrate preferably improves a throughput in the CMP in comparison with separate polishing of each surface one by one. In this case, use of a double-sided polishing apparatus for the finishing step is proposed as in JP 1-92063. Therefore, the inventers have reviewed an application of the double-sided polishing to the CMP process.
The work contacts a pad mounted on a stool during polishing whatever the polishing is the finishing step or the CMP step. JP 1-92063 inserts the work into an accommodation part in a jig (which will be referred to as a “carrier” in this application) at a predetermined fitting, and mounts them on the polishing apparatus.
As polishing proceeds, the degree of flatness of a polished surface of the work becomes higher, and an adhesion to the pad surface (polishing surface) or a frictional force increases. However, because the lower and upper stools rotate in opposite directions to one another, the work may oscillate in the accommodation part and collide with the carrier due to the frictional force and fitting, causing a chip of its edge or a generation of dust. As a result, the work may get damaged with the dust entering a space between the pad surface and the polished surface of the work, and the degree of flatness lowers. Highly precise polishing requires a dust generation preventive measure, and a prompt removal of any generated dust or a protection of the work from the dust.